This invention is directed to a method and composition for the preparation of fired pencil lead and to the resultant fired pencil lead product. More particularly this invention concerns pencil lead of high strength, prepared from a readily moldable composition which has optimum wet and dry strengths.
Known pencil leads consist essentially of graphite and clay; the ratio of graphite to clay determines the hardness of the lead. Usually, the clay, which is obtained in mining operations, is washed, that is, suspended in an excess of water and milled if necessary, mixed with graphite and processed by means of filter presses into filter cakes. The filter cakes are dried and brought to a pressable consistency with water in Z kneaders. So-called pressing aids, such as cellulose ethers or polyglycols, are added as required. These additional components generate appropriate wet and dry strengths, since the pure lead-graphite mixture, when pressed into a lead approximately 2 to 2.5 mm. thick, would be too unstable and too difficult to handle. However, these additional components have the disadvantage of exerting an effect on the wet and dry strengths, but not on the final strength of the fired lead.
In some cases, there is even interference with the ceramic bonding due to the development of excessive porosity.
It is also disadvantageous that the organic additives burn away during the firing process and lead to considerable contamination of the environment. Moreover, such additives are ineffective with respect to the rapid digestion of the clay.
In German Auslegeschrift 1,243,305, a mixture of alkaline earth bentonite, kaolin and graphite is proposed for the preparation of pencil lead. Admittedly, it was possible to do without organic additives here for improving the wet and dry strengths; however, the preparation of the mixture is very time consuming, particularly since the alkaline earth bentonite must be allowed to swell for a long time in order to achieve the desired result. Moreover, relatively high firing temperatures are necessary and compared to conventional clay formulations, the mixture is very expensive.